1. Field of the Invention
The present invention generally relates to a shift mechanism for a marine propulsion unit, and more particularly relates to an improved shift mechanism for a marine propulsion unit that shifts a transmission of the propulsion unit among, for example, forward, neutral and reverse positions.
2. Description of Related Art
Marine propulsion units such as outboard motors incorporate an engine as a prime mover to power a marine propulsion device. The propulsion device typically is a propeller and is submerged when an associated watercraft rests on a body of water. The engine typically is placed atop the outboard motor. A drive train and a transmission couple the engine with the propulsion device. Typically, the engine has a crankshaft extending generally vertically. The drive train includes a driveshaft disposed within a housing unit below the engine. The driveshaft also extends generally vertically and is connected to the crankshaft to transfer the power of the engine to a propeller shaft which also is included in the drive train. The transmission couples the propeller shaft with the driveshaft.
The outboard motor can incorporate a shift mechanism that shifts the transmission among forward, neutral and reverse positions. The propeller rotates in a direction such that the associated watercraft proceeds forward when the transmission is in the forward position, while the propeller rotates in another direction such that the watercraft proceeds backward when the transmission is in the reverse position. Otherwise, the propeller does not rotate when the transmission is in the neutral position because the propeller shaft is decoupled from the driveshaft in this position.
Typically, the shift mechanism incorporates a dog clutch assembly that operates with a gear set. The driveshaft carries a pinion of the gear set at the shaft's bottom end. The propeller shaft carries forward and reverse gears that always mesh with the pinion. The dog clutch assembly is slidably disposed between the forward and reverse gears on the propeller shaft and is coupled with the propeller shaft so as to rotate together with the propeller shaft. A mechanical shift actuator selectively connects the dog clutch assembly either with the forward gear or the reverse gear, or disconnects the dog clutch assembly from both the forward and reverse gears. Because the dog clutch assembly is coupled with the propeller shaft, the propeller shaft rotates in the forward propelling direction when connected with the forward gear, while the propeller shaft rotates in the reverse propelling direction when connected with the reverse gear. Otherwise, the propeller shaft does not rotate when the dog clutch assembly is not connected with either of the gears. The shift mechanisms incorporating such a dog clutch assembly are disclosed, for example, in U.S. Pat. Nos. 4,579,204, 5,006,084, 5,445,546, 5,556,312, 5,556,313, 5,788,546, 5,839,928 6,123,591.
The conventional shift mechanisms can shift the transmission among the forward, reverse and neutral positions in an instant because the dog clutch assembly is instantaneously connected or disconnected with the forward or reverse gear. This action, however, can produce significant shock and noise. The shock and/or noise that occurs when the transmission is shifted to either the forward or reverse positions during idle can bother or alarm (somewhat unsettle) an operator of the outboard motor or the passengers on the watercraft. The problem is more noticeable in connection with four cycle engines because the four cycle engines normally provide stable (smooth), quite operation at idle speed.